Most types of electronic gear, e.g., TV sets, aircraft navigation systems, radio transceivers and the like, incorporate circuit boards made of thin, flat dielectric sheet. Such sheet has resistors, capacitors, integrated circuits and other components mounted on it. While some boards are "hard wired" using conventional soldered wire, printed circuit boards (or "PC boards") are more common. PC boards employ flat foil strips (applied by a process akin to printing) as the "wiring" for component interconnection.
Many applications for PC boards involve hostile operating environments particularly including temperature extremes and, sometimes, rapid excursions between such extremes. And the operating environment may also include vibration which must be withstood by the PC boards. Unless recognized in board construction and testing, such environments can cause premature failure of the board per se and/or of the components mounted thereon.
To help assure that PC boards provide the requisite degree of reliability in such applications, board manufacturers often subject them to temperature tests (often referred to as "stress tests" or "stress screening") by placing them in an environmental chamber capable of producing rapid and extreme changes in temperature. For example, such a chamber might provide a temperature change of from -40.degree. F. to over 200.degree. F. in about 30 minutes. The stress test may also include vibrating the PC boards being subjected to such temperature extremes.
Stress screening culls out PC boards that exhibit what is often called "infant mortality," i.e. boards which fail prematurely under the rigors of such screening. A prominent designer and manufacturer of such environmental test chambers is Thermotron Industries of Holland, Mich.
More recently, those who make PC boards have sometimes been required to impose yet an additional form of stress--that of changing humidity. An example of an application for PC boards subjected to the additional rigors of humidity testing is aircraft electronic systems. Both commercial and military aircraft can be exposed to both very dry ambient air and very humid ambient air in a relatively short time if the aircraft moves between, say, a desert site and a jungle-like site.
Until the advent of the invention, those who test PC boards by stress screening them had three choices, none of them particularly attractive from an economic standpoint. One choice was to anticipate the need for humidity testing by purchasing (at added cost) an environmental test chamber configured with a humidity-changing capability. But relatively few PC boards are required to be tested under controlled humidity conditions--the humidity-related components may be underutilized or not used at all.
Another choice was to buy an environmental test chamber which was devoid of humidity-changing components and hope that its testing business could be sustained without having to offer that capability. And if the need to provide humidity-based testing arose, the existing chamber would be rebuilt (at substantial expense) to add a humidity control feature. If the chamber used welded seams, some of those seams had to be opened and are not easy to re-close. If not properly sealed, water gets into the fiberglass insulation behind the sheet metal--and wet fiberglass is a poor heat insulator.
A third choice is to simply purchase another chamber having a humidity control feature. In that dismaying event, one or both chambers may be grossly under-utilized and the capital investment is very substantial.
A method and apparatus overcoming some of the problems and shortcomings of the prior art would be an important advance.